1. Technical Field of the Invention
The present invention relates to an electronic circuit substrate sealed by resin.
2. Description of Related Art
An electronic circuit substrate incorporated in a device used in an outdoor environment is usually sealed with resin or the like as it is often exposed to a severe condition under an extremely high or low temperature and/or with high humidity. The substrate is usually sealed by a conventional method requiring a long time, wherein the resin is provided to the substrate under a state of liquid for coating the whole surface or the surface of a specific component of the substrate and solidified. The sealing resin thereby enables a use of the substrate under a severe external condition.
FIG. 2 is a sectional view of a circuit substrate sealed with a thermosetting resin by a conventional method. The reference numeral 1 designates a ceramic substrate, 2 represents a mounted package component, 3 represents a mounted chip component, and 4 represents a phenol resin used for sealing. In this method, the substrate 1 mounted with the components 2, 3 is dipped into the liquid phenol resin 4, and heated to solidify the resin 4.
FIG. 3 is a sectional view of a substrate sealed by injection moulding using a conventional thermoplastic resin. The reference numeral 5 designates a ceramic substrate, 6 represents a mounted package component, 7 represents a mounted chip component, and 8 represents a thermoplastic resin used for sealing. In this method, the substrate 5 mounted with the components 6, 7 is set inside a metal mould of an injection moulding device, and the thermoplastic resin 8 is injected thereto and then solidified.
However, in the above described first conventional method wherein a substrate is dipped into and coated with a phenol resin, it requires much time to thermally set the liquid resin. Also, the coating film of the resin sealing the substrate cannot be fully evened, which often hinders an automatic assembly line by a robot.
On the other hand, the second conventional method described above, wherein a thermoplastic resin is injected by an injection moulding device and solidified, requires less time for setting by application of thermoplastic resin. This method also facilitates introduction of an automatic assembly system due to more precise dimensions of the sealing resin formed by a metal mould. Since an engineering plastic has a high sealing effect, it is often used as a thermoplastic resin in this method.
However, there is a great difference in the rate of thermal expansion (linear expansion coefficient) between the engineering plastic and the ceramic substrate. Under a certain condition such as a heat cycle test or the like, the shrinking resin sometimes causes cracks on the ceramic substrate, leading to a decrease in productivity of the electronic circuit substrate. The heat cycle test is, for example, carried out within the range from -40.degree. C. to +150.degree. C. depending on each case.